The present invention relates in general to monolithic radio frequency circuits and in particular to improved monolithic voltage tunable filters.
Wireless communications products have become high volume consumer electronics accessories and are in increasing demand for a broad variety of applications. Features that are increasingly emphasized include reduced power consumption, small form factor, light weight and portability. Many of these products operate in a frequency range extending from about one hundred megahertz to about two gigahertz. As a result, there is a demand to integrate RF receivers and transmitters into high-yield silicon integrated circuit processes to allow a combination of analog, digital and RF functions on a single integrated circuit. xe2x80x9cApplications for GaAs and Silicon Integrated Circuits in Next Generation Wireless Communication Systems,xe2x80x9d by L. M. Burns, IEEE JSSC, Vol. 30, No. 10, October 1995, pp. 1088-1095, discusses examples of monolithic radio receiver and transmitter functions.
Most radio receivers and transmitters require frequency selection components that rely on some form of oscillation to provide frequency selectivity. Devices such as crystals and SAWs that employ mechanical vibration to realize frequency selection also require hermetic packages having interior cavities in order to provide reliable and robust electrical characteristics, particularly in view of environmental hazards. Often, the package is more expensive than the component within it. Additionally, multiple packages are required, because the materials useful for these types of frequency selection components do not support active electronic devices, and vice versa. Further, devices that rely on mechanical oscillation also use materials having different packaging requirements than do active electronic devices.
Electronic circuits that do not rely on mechanical vibrations for frequency selection characteristics often rely instead on electrical resonances to provide frequency selectivity. Practical electrical resonators in this frequency range require a combination of capacitance and inductance. Of these, inductance is particularly difficult to realize in compact form together with reasonably high quality factor, or xe2x80x9cQ.xe2x80x9d Q is often defined as the amount of energy stored divided by the amount of energy dissipated per cycle, but can also be defined as a center frequency divided by a three dB bandwidth of a frequency response. The latter definition is used herein in instances where the former is inapplicable.
Known approaches for realizing monolithic inductance include spiral inductors, transmission lines and bond wires. For example, xe2x80x9cA 1.8 GHz Low-Phase-Noise Spiral-LC CMOS VCO,xe2x80x9d by J. Cranickx and M. Steyaert, 1996 Symp. on VLSI Cir. Dig. Tech. Papers, pp. 30-31 describes a spiral inductor approach that achieves a Q of 5.7 near two gigahertz. xe2x80x9cIntegrated Passive Components in MCM-Si Technology and their Applications in RF-Systems,xe2x80x9d by J. Hartung, 1998 Int. Conf. on Multichip Modules and High Density Packaging IEEE Cat. No. 0-7803-4850-8/98, pp. 256-261, reports Qs and their frequency dependence for spiral inductors vs. substrate resistance, with highest Qs and self-resonant frequencies for spiral inductors fabricated on higher-resistivity substrates. A recent overview of spiral inductive components, entitled xe2x80x9cAnalysis, Design, and Optimization of Spiral Inductors and Transformers for Si RF IC""s,xe2x80x9d by A. Niknejad and R. Meyer, IEEE JSSC, Vol. 33. No. 10, October 1998, pp. 1470-1481, gives examples of Qs having peak values around five and inductances of up to about ten nanoHenrys for spiral inductors fabricated on silicon.
Transmission line approaches to realizing monolithic inductance tend to be bulky and relatively lossy in this frequency range. Bond wires can provide Qs ranging from 11 to 15, as described in xe2x80x9cA 1V, 1.8 GHz, Balanced Voltage-Controlled Oscillator with an Integrated Resonator,xe2x80x9d by D. A. Hitko et al., Proc. Symp. Low Power Electr. and Des., pp. 46-51 (1997). Bond wire inductors tend to be relatively large compared to other integrated circuit components, but do permit the surface area beneath them to be used to fabricate other integrated circuit elements prior to bond wire installation. Bond wire inductors also require bond pads, which are relatively large and which also preclude use of their area for other purposes. None of these approaches provide the combination of small form factor, high Q and packageability needed for many applications.
Another approach to providing a frequency selection function in monolithic form relies on impedance transformations that are possible with active circuits, i.e., circuits including transistors. U.S. Pat. No. 5,175,513, entitled xe2x80x9cOscillator Circuit Employing Inductive Circuit Formed of Field Effect Transistorsxe2x80x9d and issued to S. Hara, describes an example using MESFETs. U.S. Pat. No. 5,726,613, entitled, xe2x80x9cActive Inductor,xe2x80x9d issued to H. Hayashi et al. and xe2x80x9cA Novel Broad-Band MMIC VCO Using an Active Inductor,xe2x80x9d H. Hayashi and M. Maraguchi, IEICE Trans. Fundamentals, Vol. E81-A, No. 2, February 1998, pp. 224-229, describe similar approaches. While these approaches do provide compact circuits, they use GaAs MESFETs, which are not as manufacturable as CMOS FETs and which are not cost-competitive with silicon integrated circuits. Additionally, it is much more expensive to provide complex ancillary functions on GaAs substrates, such as may be realized using digital circuitry, than is the case with silicon substrates.
Therefore, there is a need for monolithic circuitry that provides frequency selection functions and that is compatible with cost-effective approaches to providing other circuit functions.
In one aspect, the present invention includes an active frequency selection circuit. The frequency selection circuit includes a first port presenting a frequency-dependent impedance and a first amplifier configured to provide nearly unity gain, a high input impedance and a low output impedance. The first amplifier has a first input and a first output. The first output is coupled to the first port. The frequency selection circuit includes a first phase shifter that, in one aspect, is formed by a first capacitor coupled between the first port and a reference voltage. The frequency selection circuit also includes a second amplifier configured to provide greater than unity voltage gain, a high input impedance and a low output impedance. The second amplifier has a second input coupled to the first output and a second output coupled to the first input through a second phase shifter. In one aspect, the second phase shifter is formed from a second capacitor coupled between the second output and a reference voltage. The first and second amplifiers and the first and second phase shifters are coupled in a loop such that the frequency dependence of an impedance presented at the output of the first amplifier emulates the impedance of a parallel RLC tank circuit. The circuit is expected to provide a Q of greater than fifty at a center frequency xcfx89o of about five hundred megahertz. Significantly, the active frequency selection circuit may be formed as a monolithic CMOS or a BiCMOS integrated circuit. As a result, the active frequency selection circuit may be combined with other kinds of circuits, such as signal processors and digital circuits.
Voltage dependent values for parasitic impedances and admittances of the first and second amplifiers cause the center frequency xcfx89o to vary linearly with supply voltage. As a result, the center frequency xcfx89o can be swept over a range of tens or hundreds of megahertz by altering the supply voltage.
In another aspect, the frequency selection circuit forms a tuned transimpedance amplifier, with a second port coupled to the second output of the second amplifier. The second port forms an output for the transimpedance amplifier. The center frequency xcfx89o of the tuned transimpedance amplifier also may be varied simply by varying the supply voltage.